The present invention relates to a system with portable data media with decentralized verification of inputs and outputs as well as transfer of data and memory contents of data, completed under security via a user-controlled interface. Such a system uses a portable terminal which has a receiving means for a portable data medium, in particular a bank card, and it is possible to execute with the terminal different operations with the data medium. For this purpose the data medium comprises at least one control circuit and an electronic memory. The simplest function is the display of the memory content, that is the content of specific memory locations for which purpose, apart from means for establishing an operative connection with the data medium, the terminal comprises also a display device and an energy source. The operative connection can comprise contacts, but connection can also be realized free of contacts via an electric or electromagnetic field. In the case of a bank card or a telephone card, a sum of money available on the card or a number of available value units can be displayed in this way. Furthermore, small portable terminals are known which, apart from the display device, also comprise control elements in the form of buttons and, through specific actuations of the buttons, data on a data medium connected with the terminal can be changed, for example they can be moved, for example, from one storage location to another.
The European Patent Application 97 100 003.9 describes a data medium which comprises at least two memories or memory locations, wherein into one of the memories data representing, in particular, sums of money, can only be written and from at least one further memory sums of money can only be removed, that is, debited. In order to transfer, in the case of such data medium, sums of money, respectively corresponding data, from the write-only memory into the at least one debitable memory, the data medium is connected with a small portable terminal which comprises an energy source, a display device and control elements, which for such transfer must be operated in a predetermined manner. The terminal is therein advantageously and uniquely assigned to a specific data medium, which assignment can be verified by comparing check data stored in the data medium and in the terminal. This has the advantage that a finder or a thief of such data medium cannot dispose of the amount of money stored therein if he or she does not also have the associated terminal. Establishing such fixed assignment between data medium and terminal, however, is not always easy. For example, the site which issues the data media can also supply the terminal, and both must be correspondingly programmed before they are issued. But such programmability is complicated especially for the terminal. Still more difficult is establishing the assignment if the terminal is to be issued independently of the data medium. In addition, the terminal, as a function of the desired application of the data medium, must comprise a more or less complex control circuit for carrying out data changes in the data medium so that for different types of data media also different terminals are required.